The foregoing single step oxidation of liquid o-xylene is readily seen as superior to the oxidation of o-xylene vapor with air in the presence of solid particulates of vanadium oxide based catalyst which produces phthalic anhydride in 70 mole percent yield under non-explosive xylene to air concentrations and up to 78 mole percent yields with xylene to air concentrations well within the explosive range. Such oxidation of o-xylene in the vapor phase also results in the substantial over-oxidation of o-xylene to large amounts of oxides of carbon together with smaller amounts of maleic anhydride, citraconic acid and benzoic acid.
In contrast, in the conduct of the present invention the oxides of carbon produced amounts to 5 to 10 mole percent of the o-xylene but some of the oxides of carbon are from the co-production of 1 to 2 mole percent benzoic acid. Most of the o-xylene not so over oxidized, 3 to 7 mole percent, and not going to product appears as aromatic oxygen-containing co-products having boiling points well above the boiling temperature of o-phthalic acid and its anhydride.
U.S. Pat. Nos. 2,696,499 and 2,833,816 also describe a substantially solventless catalytic liquid phase oxidation of xylene at elevated temperature and pressure. The process of the first patent uses a xylene soluble cobalt compound (e.g., cobalt naphthenate, cumate, toluate, etc.) as catalyst and limits the oxidation to 10-50 percent conversion of xylene to toluic acid to suppress formation of unwanted oxygenated derivatives not precursors of phthalic acid. Such limited conversion also produces phthalic acid product which is separated (e.g., by filtration) and the filtrate is distilled to remove precursors of toluic and phthalic acids and reject the unwanted non-precursors. Said precursors are returned to the oxidation with fresh xylene and catalyst.
The second patent uses a polyvalent metal (e.g., cobalt, manganese or mixture thereof) with a source of bromine as the catalyst. Such xylene oxidation results in a 20-30% conversion of the xylene to phthalic acid product and the remainder to mainly toluic acid. The phthalic acid product is separated from toluic acid which is added to fresh xylene and catalyst and charged to the oxidation step.
U.S. Pat. No. 3,920,735 describes a solventless air oxidation of o-xylene at 205.degree. C. and gauge pressure of 21 kg/cm.sup.2 using for each gram mole of xylene 3 milligram atoms of each of cobalt, manganese and zirconium and 9 milligram atoms of bromine from hydrogen bromide. The reaction product comprised the following products in the molar yields based on the o-xylene charged: phthalic anhydride, 61 mole %; o-toluic acid, 0.97 mole %; phthalide, 3.95 mole %; 2-carboxybenzaldehyde, 1.88 mole %; and 0.1 mole % o-xylene. Also 10.5 mole percent of the o-xylene was burned to oxides of carbon and water.
The prior solventless catalytic liquid phase oxidation of o-xylene closest to the present inventive liquid phase oxidation is disclosed in British Pat. No. 856,245 published Dec. 14, 1960. Such solventless process was conducted at a temperature of 150.degree. C. in the presence of a mixture of cobalt and maganese bromides as catalyst, at atmospheric pressure, and in an apparatus equipped with a water removal decanter. But said oxidation ceased when 70 mole percent of the o-xylene had been converted to phthalic anhydride. However, the 30 mole percent partially oxidized products when combined with both fresh o-xylene and catalyst is reported as proceeding upon further oxidation only to a tar-like product rather than to phthalic anhydride. It is further reported in said British Patent that small quantities of such tar-like product when added to a large quantity of o-xylene would inhibit its oxidation.
To overcome such limitations on solventless oxidation of o-xylene the patentee of the British Patent devised a two step oxidation wherein the first step is conducted in the presence of catalysis provided by only a polyvalent metal oxidation catalyst (i.e., no or very low ratio of bromine to xylene) until oxygen consumption ceases; and the second step is conducted also until oxygen consumption ceases in the presence of catalysis provided by both the polyvalent metal oxidation catalyst and a source of bromine with the product of the first step free or freed of unchanged o-xylene. Such process is illustrated by use of 20 hour reactions in each of the two steps conducted at a temperature of 150.degree. C., at atmospheric pressure and in apparatus equipped with a water removal decanter. By such 40 hours of reaction and without recycle of intermediate oxygenated products, phthalic anhydride was produced in a 40 mole percent yield and 98.8% purity.
The present inventive process not only avoids the formation of oxidation inhibiting tar-like co-products but also in a single oxidation step without recycle of precursor intermediate oxidation products can produce 85 to 95 mole percent yields of impure o-phthalic acid readily converted to partially purified phthalic anhydride of 98% purity.
Such results are indeed suprising in view of the fact that the present inventive one step oxidation uses both the polyvalent metal oxidation catalyst and bromine in an amount substantially greater than used in the first step of the two step process of the British Patent.